It is known that multi-stage emulsion polymers made significant improvements over previous mixtures and blends such as described in U.S. Pat. No. 4,916,171. By polymerizing one component in the presence of another to form a multi-stage polymer, improvements in stability, rheology and water resistance were realized.
It is still further known that the addition of an acrylic polymer into cement mixtures improves mortar properties such as strength and adhesion of the mortar to a variety of substrates such as cement, wood and polystyrene form. However, while the addition of multi-stage emulsion polymers have made some improvement in strength and adhesion properties, further improvements are required.
General physical properties such as strength and adhesion of the mortar or modified cementitious material generally coincides with the polymer stiffness, the glass transition temperature (Tg) and the minimum film forming temperature (MFFT). Generally, the higher the Tg and MFFT of the polymer used in cement mixtures, the greater the strength and adhesion of modified mixtures (Ohama, et al., ACl Materials Journal, vol. 88, pp. 56-61 (1991). MFFT is the temperature at which a latex polymer forms a continuous film. However, the disadvantage in employing a high Tg polymer as the modifier is the need for higher curing temperatures. A higher curing polymer is not a concern where elevated curing temperatures for the most part are obtainable. However, the need for higher curing temperatures significantly limits the use of high Tg polymers as modifiers during the autumn or early winter periods of the year. Lower ambient temperatures during these periods significantly limits the use of high Tg polymers. On the other hand, lower Tg latex polymers may be otherwise preferred since shorter curing times and lower curing temperatures make their use practicable during the winter and fall when ambient temperatures are lower.
Another inherent problem with the use of lower Tg polymers is the loss of strength and adhesion of the polymer-modified material. Therefore, what is required is a latex polymer that may be useful as a modifier for cementitious material well into the fall and early winter when ambient temperatures are generally cooler. The use of lower Tg polymers at lower temperatures must be obtained without adversely effecting strength and adhesion of the modified material.
In the further evaluation of multi-stage polymers as modifiers for cementitious materials, we found that compositional modification of the ionically-soluble polymer and the method for preparing the same provides an emulsion modifier for cementitious materials that cures at lower temperatures, while the strength and adhesion properties of the modified material is uncompromised. Also, the lower curing temperature is obtained without other properties of the polymer-modified material, such as, tensile strength being adversely effected.
The current invention discloses a novel multi-stage emulsion polymer produced by a sequential emulsion polymerization process that when used as a cement modifier improves physical properties, such as strength and adhesion, while other properties such as tensile, flexural and compressive strength of the cement are maintained.